The present invention relates to a hip joint prosthesis of the type having a hollow shaft provided with locally weakened sections.
A prosthesis of this type is disclosed in DE-OS [German Laid-open application]No. 2,851,598. One drawback of this known prosthesis shaft is that it has high strength, or rigidity, with respect to the surrounding bone regions so that the shaft, because of its rigidity, constitutes a foreign body which performs movements relative to the bone that might lead to bone resorption.
Also known are perforated accessory elements for shaft prostheses, which elements serve as additional supporting elements to fix the shaft prostheses when an adhesive or bone cement is used, as disclosed in DE-OS No. 2,617,749. In this prosthesis, however, the strength or rigidity, of the entire arrangement is determined essentially by the prosthesis shaft and not by the additional supporting element. Moreover, this type of structure is extraordinarily complicated and hence expensive.
In principle, the good initial results after total hip joint replacement are presently sometimes impaired by the still unresolved problem of the cemented endoprosthesis coming loose. The natural transfer of forces from the femoral head via the femoral spur [calcar femorale] into the shaft of the femur is considerably interfered with by the installation of an endoprosthesis. The installation of a prosthesis having a collar has the drawback that nonphysiological pressure forces are generated locally at the prosthesis seat. A prosthesis without collar leads to nonphysiological circumferential stresses in the calcar femorale and in the coaxial femur region due to the wedge effect of the conical prosthesis shaft. Both the locally high pressure forces and the circumferential stresses lead to resorption of bone in the region of the calcar femorale. Due to this resorption, high circumferential stresses are also generated at a prosthesis having a collar because of the lack of support by the calcar femorale and, after a certain period of time, breaks will occur in the cement. The prosthesis thus becomes free in its upper region and moves with increasingly larger bending deformations in its cement quiver, with the result that the prosthesis is loosened completely or breaks.